The present invention relates to tethering components such as components for mooring a floating or submerged device or structure in a body of water. The components are particularly suitable for mooring applications where a small footprint and low scope operation are required.
Traditionally mooring components have been limited to near-shore use, for example tethering boats or pontoons to a pier or quay. Conventional mooring ropes are usually made from synthetic materials, such as polyester, nylon or Kevlar®. Although polyester and nylon mooring ropes are quite elastic, they can only deliver small elongations of the order of 10-25%. Conventional mooring ropes may also be made from wire filaments, which are extremely strong, but difficult to handle and maintain. A conventional mooring component made from a combination of wire rope and synthetic materials is often referred to as a hawser.
More recent advances in rope and cable technology have also seen the use of polymer materials between steel strands to help protect the ropes from fatigue in mining and oil/gas applications. Such protective sheaths of polymer material, for examples in many of Bridon's Dyform® ropes, do not make use of the polymer's elongation ability, as the elongation of the cable is limited by the steel strands. U.S. Pat. No. 4,534,262 and U.S. Pat. No. 4,597,351 contain examples of such a protective sheath approach. The strong sheath material can be braided like a rope but it is therefore also limited like a rope with similar maximum extensions. These maximum extensions depend on the braid design but are very limited and do not make use of the 100%+ extensions possible with an elastomeric material. Using existing braided nylon and polyester ropes would bring more benefit delivering higher load capacity for the same elongations. Furthermore the braiding itself becomes a wear issue on these types of designs suffering from the same wear problems that synthetic ropes have under cyclic load environments.
FR2501739 contains an alternative approach for a towing line, where a non-elastomeric bypass cable with a longer length than the core rubber section is used for protection from high loads. In this scenario the rubber core can now stretch to a far longer length before the steel bypass cable takes the load. The steel cable itself however is non-elastic and has an almost infinite slope (stress/strain) compared to the elastic core. This causes a significant problem with shock loads. Once the rubber core is stretched to its limits the steel cable protects it but high shock loads are generated causing higher peak loads and requiring thicker steel cables than may otherwise be desired. These high shock loads increase the anchor loads and the load on the device itself increasing fatigue damage and costs.
Seaflex® is an elastic mooring system for securing pontoons. The mooring component is a hawser comprising one or more rubber strands and a so-called bypass cable formed of stiff synthetic fibre or wire that prevents the rubber strand(s) from over-extending. The Seaflex® rubber hawser can withstand a force greater than 10 kN and more than 100% elongation to allow the mooring to take care of a degree of water level fluctuation. US2005/103251 and US2009/202306 are similar to the Seaflex approach and describe elastomeric mooring solutions where a steel bypass cable (“safety-locking loop”) is used. In these cases a problem again occurs when the elastomeric ropes are fully extended and the steel cable engaged. Due to the almost infinite slope of the steel cable compared to the elastomeric ropes, high shock loads are created which can lead to fatigue, damage and higher anchor costs. In general these conventional steel bypass mooring solutions are only suitable for low load, near shore, sheltered applications, usually with multiple hawsers sharing the load.
However, conventional mooring solutions such as hawsers are not suitable for tethering devices to the seabed in deep water or for mooring in environments where the floating device is subject to large tidal currents and/or wave motion. Off the north coast of Scotland, for example, in a water depth of 40 m the waves will on average be less than 2 m high, increasing to up to an average 4 m during annual storms and an average greater than 5 m in a 100 year storm. The individual waves can be many times higher than the average, leading to changes in wave height of a significant fraction of the wave depth (low scope scenario). Conventional cables and hawsers either do not have the strength to withstand the forces imposed on a floating device by tidal movement and unpredictable storm waves, or else cost far too much to be able to install a system which can handle these forces.
There is therefore required another class of mooring components that can be used to tether floating devices and sea-based structures such as renewable energy devices, including wave energy conversion devices, tidal turbines and tidal platforms, fish farms, oil rigs and off-shore wind farms, especially in low scope or high variability environments. In these environments it is desirable to have a mooring solution which can deliver a low slope (ideally flat) load response under normal wave or tidal response, with a smoothly engaging high slope protective elastomeric response under more extreme environments. Ideally this engaging higher slope response would be non-linear with a continuing increase in slope with extension.
The main purpose of a mooring component is to control relative movement between the device being moored and its tether point. Such movement may be caused by wave and/or tidal motion. The mooring component must therefore apply a restoring force against movement of the device. It can be difficult to meet the demands on a mooring component where the device to be moored experiences relatively large displacements relative to the depth of water. In these environments it is desirable that the scope of the mooring is not too large, where the “scope” is defined as the length of mooring per unit of water depth. It is also desirable to minimise the footprint of the mooring system, where the “footprint” is the seabed area occupied by the mooring component.
FIG. 1 is a schematic diagram of a basic single point catenary mooring conventionally used to tether a floating structure 3 such as a tidal platform. The catenary mooring line comprises a free hanging line or cable 5, typically a steel chain, running horizontal to the seabed. The restoring force of the mooring line 5 is primarily generated by the hanging weight and pre-tension in the line. FIG. 1 shows that as the water depth increases due to large waves, the catenary chain 5 is lifted off the seabed 4 as the platform 3 drifts upwards and to the left. As the water depth decreases, the chain 5 is laid along the seabed 4 and the platform 3 drifts downwards and to the right. Thus very large amounts of chain and a large space envelope is required to allow horizontal movement of the platform as the water depths rise and fall. This results in very high material costs for the mooring system and restricts the positioning of the platform in an array. Catenary mooring systems can be used even in deep sea applications but the chain must be made so long that it does not exert any vertical load at the anchor point.
Due to the horizontal load reacting nature of the conventional drag embedded anchors which are used with catenary systems, the scope of the cable must be chosen such that the cable is never entirely picked up from the seabed for the given environmental conditions. Large waves can be up to 20 m high, i.e. the same order of magnitude as the water depth, and the length of chain required to deal with such changes then becomes very large. Normally a scope of three suffices, but in shallower water a scope of more than five is frequently required. Such a mooring system is often inefficient and takes up a lot of seabed space around the device, resulting in high costs and a large footprint. In the most extreme conditions the horizontal mooring force on a steel catenary system can be greater than 5000 kN. A further disadvantage of a catenary system is fatigue, as the mooring lines tend to wear at the seabed touch down point.
Accordingly there are a number of problems when it comes to implementing a catenary mooring system with a tidal platform or the like. In particular, very large scopes, seabed footprints and horizontal motion envelopes are required to allow the platform to ride the waves.
Alternative mooring systems do exist which can be more suitable to specific environments, such as using surface floats, or weights. These systems however also result in considerable additional cost and often suffer from similar problems of larger footprints and high forces. Many of these alternative approaches will use both steel cable and polyester ropes to try to overcome the challenges, but they cannot provide an adequate response to the movement of bodies in highly variable marine environments. Where they particularly suffer is in high peak forces or in large variations in force over time, resulting in higher fatigue.
As an alternative to catenary mooring systems, a limited number of elastic mooring components have become available which are taut as compared to a catenary system. As mentioned above, these cables usually comprise an elastomeric e.g. rubber material so as to allow the mooring to elongate to accommodate movement of a device, for instance due to tidal currents. In these mooring components one or more rubber strands may be combined in parallel with a so-called bypass cable formed of stiff synthetic fibre or wire that prevents the rubber strand(s) from over-extending. Such bypass cables however have a significant problem in that a typically non-smooth stress-strain response risks very high peak forces in response to elongation, causing fatigue and damage.
Mooring components comprising an elastomeric material are becoming popular in near shore and dock mooring applications. They provide a number of advantages over traditional mooring solutions by allowing a flexible component in the mooring system to stretch with the heave and surge of the vessel or device. They also cause less seabed damage, as additional slackness can be built into the mooring system. However, these mooring systems are principally designed to prevent drift of vessels and are not designed to provide low scope, small footprint performance in deeper waters. Current elastomeric solutions only work well where the change in wave height is small with respect to the depth of water in which the mooring is used, such as in-harbour pontoons, or in estuaries where tidal changes in water height are low.
Elastic mooring lines that comprise rubber elements and a stiff bypass cable to prevent over-extension are limited in the lengths to which they can be made, as the synthetic fibre or steel bypass cable can add disproportionately to the weight of the component. In practice such lines are no more than about 10 m long and therefore find most use in mooring pontoons and boats in a marina. The braided synthetic ropes in some of these moorings can also suffer from wear problems.
Furthermore these elastomer solutions all suffer from the same fundamental problem, namely that the diameter of elastomeric material required to deliver a restoring force in low wave scenarios is much smaller than the diameter required to withstand high forces. For normal rubber material, a counter force of ˜MN as needed in high sea states would require material diameters >1 m. This diameter would exist along the entire length of the rubber component, resulting in unmanageable or uneconomic components. This therefore restricts the range of non-linear force response which can be delivered from conventional elastomer components to much smaller ranges, which cannot address the mooring needs in non-sheltered e.g. high wave environments. A steel bypass cable can of course deliver such force with a smaller diameter but if such a cable is included then the force response will not be smooth.
WO 2011/033114, published after the priority date of the present application, discloses a solution to this problem. It proposes using multiple different elastomer lengths with thicker and thicker elastomers, delivering higher and higher load protection, engaging at long and longer extensions. While this solution does indeed work, it suffers from the same problem highlighted above, namely that the thickness of elastomer required to withstand the high loads becomes very large. Furthermore, the thickest elastomers are also the longest elements in the component and therefore the entire device becomes unmanageable at larger sizes.
Although the currently available elastic lines such as Supflex® may be able to withstand severe weather conditions in sheltered environments without breaking, they provide a steeply increasing stress-strain response upon elongation and may therefore apply relatively high forces on the mooring system. While they may provide a non-linear stress-strain response to applied wave forces, they do not deliver the smooth performance and response curves required for more challenging mooring environments. In order to achieve the level of performance required for many offshore applications, a relatively large scope, that is, length per unit of depth and a large seabed footprint would be required with these moorings. This means that more material, or higher-grade material, would have to be used and the cost may become prohibitive.
Ideally, a deep sea mooring system needs to be adaptable to the sea states at the location at which it is placed and so it must be able to adjust its response to the applied forces from the waves over very short time periods. Ideally, such a mooring system is self-adjusting so that risk of failure in harsh environments is reduced. Ideally, the mooring system should absorb load forces at the lowest possible breaking limit. It should also be cost-effective.